Ross Formation - UPPER CARBONIFEROUS DEEPWATER SEDIMENTS

The Ross Formation is found outcropping along the shores of the Shannon Estuary from Leck Point on its south shore to Loop Head to the north, and then around the Atlantic flank of this peninsula to Fisherman’s Point. These exposures of this formation range from cliffs that can be viewed from a boat or small plane to rocky foreshores that are accessible from the nearby roads and fields. The quality of these outcrops makes it possible to examine the principal architectural elements of deepwater lobes and channels that form the Ross Sandstone. Similar outcrops of the underlying basinal Clare Shale, and the overlying unstable slope sediments of the Gull Formation and the capping shallow water Tullig cyclothems are accessible too.

	Goniatite Zones

Pyles (in press) uses seven the condensed sections, initially described by Hodson (1954a, & b), and Hodson and Lewarne (1961), to subdivide the Ross Sandstone into six stratigraphic cycles. These cycles range from 0 to 190 m (0 to 623 ft) and have an average thickness of 80 m (260 m). He calculates that the cycles represent between 200 and 250 ky each. Using ties from all the goniatite-bearing condensed sections occurring in this part of western Ireland, published data and his own compilations, Pyles (in press) has constructed four regional stratigraphic cross sections of the Ross Sandstone and Clare Shale recognizing that the offshore the data west of the Loop Head Peninsula is the most circumstantial.

	Multiple Cross Cross Sections across Shannon Basin

Pyles (in press) has established that the thickest part of each stratigraphic cycle is elongated and parallel to the axis of the Shannon Estuary. The thickest part of each condensed-section bound stratigraphic cycle occurs near Loop Head Peninsula, and strata in these cycles thin away from that site. Using the data of Lees (1961) and Strogen et al. (1996) he confirmed that Visean (Early Carboniferous) carbonates have the same patterns of thickness. Additionally the boundary between shallow and deepwater Visean carbonates documented by Strogen et al. (1996) matches the margin of the basin defined by thickness data of the Ross Sandstone. This sandstone is thick above deepwater Visean carbonates and thin or nonexistent above the shallow water Visean carbonates. Thus the Ross Sandstone inherited a basin physiography that matches that of the Visean carbonates and the Iapetus Suture of McKerrow and Soper (1988), (Pyles, in press).

	Carboniferous Iapetus Suture

The vertical aggradational regional stacking patterns of the depocenters in the Ross Sandstone and Clare Shale show that the Ross Sandstone overlies and laterally interfingers with the Clare Shale (see cross section). The sediment thins to zero within the limits of the basin suggesting that the accumulation rates slow here, with an exception being the localized thick located south of the basin in the R1a2-R1a3 interval. This anomaly is thought to record a structural event that was limited to this time frame. Pyles, (in press) interpreted this to indicate that the Ross fan was confined to the Shannon basin and is a “closed” depositional system. This is supported by the lack of Ross Sandstone-aged sediment in the Dublin Basin (Ramsbottom et al., 1978). Pyles, (in press) interprets the fan-like patterns for each stratigraphic cycle as radiating away from the site of the present day Dingle Peninsula. Using core data Tate and Dobson (1989) and Tate (1993) record a middle Namurian unconformity on the eastern margin of the Porcupine basin. Pyles, (in press) believes this implies that the Ross Sandstone and Clare Shale correlate in the landward direction to an unconformity, interpreted to record erosion and sediment bypass during deposition of the Ross Sandstone.

The character of the Ross Sandstone exposures and their subdivision by the condensed sections containing goniatites make it is possible to use sequence stratigraphic approach to determine the depositional origins of the component sands and shales. It is possible to use the framework provided by the bounding surfaces of the various genetically related component lithofacies, their geometries and the hierarchies and their stacking patterns of these geometries to establish their depositional order and setting. Examining these cycles he has interpreted to the Ross Sandstone to be composed four architectural elements (1) channels, (2) lobes, (3) slumps, (4) 12 mudstone sheets. The dominant architectural element proved to be the lobe while channels occupy a smaller percent of the strata. Slumps only occur in the upper Ross Sandstone.

We have compiled a gallery of photographs of the Ross Formation starting at Leck Point, along the shores of the Shannon Estuary to Loop Head including the cliffs at Kilcredaun, Rehy, and Kilcloher, the foreshore exposures at Kilbaha and Dunmore Bays, the cliffs at Loop Head; the foreshore exposures at Ross Bay, and Bridges of Ross and the cliffs at Gull Island and Fisherman’s Point, Trusklieve, Foohagh Point, Diamonds Rocks and Spanish Point.

Useful References

Chapin, M.A., Davies, P., Gibson, J.L. & Pettingill, H.S. (1994), Reservoir architecture of turbidite sheet sandstones in laterally extensive outcrops, Ross Formation, western Ireland. In Weimer, P., Bouma, A.H. & Perkins, R.F (eds), Submarine fans and turbidite systems, GCSSEPM Foundation 15th Annual Research Conference, 53-68.

Collinson, J.D., Martinsen, O. Bakken, B. and Kloster, A. (1991), Early fill of the western Irish Namurian Basin: a complex relationship between turbidites and deltas. Basin Research, 3, 223-242.

Davies, S.J. & Elliott, T. (1996), Spectral gamma ray characterisation of high resolution sequence stratigraphy: examples from Upper Carboniferous fluvio-deltaic systems, Co. Clare, Ireland. In Howell, J.A. & Aitken, J.F. (eds) High resolution sequence stratigraphy: innovations and applications. Special Publication of the Geological Society London, 104, 25-35.

Gill, W.D. (1979), Syndepositional sliding and slumping in the West Clare Namurian Basin, Ireland. Geological Survey of Ireland Special Paper 4, 31pp.
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Hodson, F., 1954a, The beds above the Carboniferous limestone in north-west County Clare, Eire: Quarterly Journal of the Geological Society of London, v. 109, p. 259-283.

Hodson, F., 1954b, The Carboniferous rocks of Foynes Island, County Limerick: Geological Magazine, no. 2, p. 153-160.

Hodson, F. & Lewarne, G.C. (1961) A mid-Carboniferous (Namurian) basin in parts of the counties of Limerick and Clare, Ireland. Quart. Geol. Soc. Lond., 117, 307-333.

Martinsen, O.J. (1989), Styles of soft sediment deformation on a Namurian (Carboniferous) delta slope, western Ireland Namurian Basin, Ireland. In Whatley, M.K.G. & Pickering, K.T. (eds) Deltas: sites and traps for fossil fuels, Geological Society Special Publication, 41, 167-177.

Martinsen, O.J. & Bakken, B. (1990), Extensional and compressional zones in slumps and slides in the Namurian of County Clare, Ireland. Journal of the Geological Society, 147, 153-164.

Pulham, A.J. (1987), Depositional and syn-sedimentary deformation processes in Namurian deltaic sequences of west County Clare, Ireland. Unpublished PhD thesis, University of Wales, Swansea.

Pulham, A.J. (1989), Controls on internal structure and architecture of sandstone bodies within Upper carboniferous fluvial-dominated deltas, County Clare, western Ireland. In Whatley, M.K.G. & Pickering, K.T. (eds) Deltas: sites and traps for fossil fuels, Geological Society Special Publication, 41, 179-203.

Pyles, D.R. (in press), Stratigraphic architecture of a structually confined submarine fan, Carboniferous Ross Sandstone, Western Ireland, Bulletin of the American Association of Petroleum Geologists.

Rider, M.H. (1969), Sedimentological studies in the West Clare Namurian Basin, Ireland and the Mississippi Delta. Unpublished PhD thesis, Imperial College London.

Rider, M.H. (1974), The Namurian of West County Clare. Proceedings of the Royal Irish Academy, 74B, 125-142.

Rider, M.H. (1978), Growth faults in the Carboniferous of western Ireland. Bulletin of the American Association of Petroleum Geologists, 62, 2191-2213.

Sevastopulo, G.D. (1981), Upper Carboniferous. In Holland, C.H. (ed.) A geology of Ireland. Scottish Academic Press, 173-199.

Wignall, P.B. and Best, J.L. (2000), The Western Irish Namurian Basin reassessed. Basin Research 12, 59-78.

Wignall, P.B. and Best, J.L. (2002), The Western Irish Namurian Basin reassessed – a discussion. Basin Research 14, 523-542.